Liquid Crystal-Templated Synthesis of Mesoporous Membranes with

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Liquid Crystal-Templated Synthesis of Mesoporous Membranes with Predetermined Pore Alignment Asli Karausta, and Emre Bukusoglu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14121 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Applied Materials & Interfaces

Liquid Crystal-Templated Synthesis of Mesoporous Membranes with Predetermined Pore Alignment Aslı Karausta and Emre Bukusoglu*

Chemical Engineering Department, Middle East Technical University, Ankara 06800, Turkey. e-mail: [email protected] Keywords: Liquid crystals; templated synthesis; polymerization; mesoporous films; ultrafiltration

Abstract

We demonstrate that polymeric films templated from liquid crystals (LCs) provide basic design principles for the synthesis of mesoporous films with predetermined pore alignment. Specifically, we used LC mixtures of reactive (4-(3-acryloyoxypropyloxy) benzoic acid 2-methyl-1,4phenylene ester (RM257)) and non-reactive (4-cyano-4!-pentylbiphenyl (5CB)) mesogens confined in film geometries. The LC alignment was maintained by functionalization of the surfaces contacting the films during polymerization. Through photopolymerization followed by extraction of the unreacted mesogens, films of area in the order of 10 cm was obtained. We found that, when 2

restricted to an area either through a mechanical or a configurational constraint, open and

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accessible pores were incorporated into the films. The average direction of the pores could be determined by the LC director during polymerization, and the average diameter of the pores can be tuned in the range of 10-40 nm by varying the reactive monomer concentration. The polymeric films synthesized here can potentially be used for the ultrafiltration purposes. We demonstrated successful separations of proteins and nanoparticles from aqueous media using the polymeric films. The films exhibitted two orders of magnitude higher flux when the pores were aligned parallel to the permeate direction compared to the perpendicular direction. Overall, the outcomes of this study provide basic tools for the synthesis of porous polymeric films with predetermined pore directions that can potentially be suitable for separations, drug delivery, catalysts, etc.

1. Introduction Liquid crystals (LCs) have found recent interest in the development of soft, functional materials.

1

Beyond their use in displays, thermometers, or sensors that take advantage of their optical properties, LCs that combine unique long-range ordering and liquid-like flow properties have been developing as a tool for the synthesis of polymeric materials that would not be possible with their isotropic counterparts. Thermotropic and lyotropic LCs have been used extensively in these 2–7

studies. Thermotropics exhibit LC phases as a function of temperature, whereas lyotropics exhibit LC phases as a function temperature and chemical composition. To date, successful studies have been achieved in different forms. For example, polymerdispersed (PDLC) and polymer-stabilized LCs

8–12

were formed through polymerization of

monomers within a non-reactive LCs to influence the response of the LCs to external fields. Within majority of these studies, non-mesogenic reactive monomers were used in LCs.

13,14

Alternatively,

difunctional and/or monofunctional mesogenic monomers have been used to form LC elastomers (in forms of droplets or films) to provide responsive properties to the polymeric product.

2–6,15–18

More


2

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recently, reactive and non-reactive mixtures were polymerized in LC droplets formed in aqueous emulsions to provide internal structuring to the polymeric microparticles.

7,19–23

Due to the anisotropic

shrinkage upon the extraction of the unreacted mesogens, LC droplets maintain various shapes such as spindle, spherical, tear drop and spherocylindrical shapes dependent on the initial droplet configuration. More interestingly, spherical microparticles exhibit minimal shrinkage compared to the other shapes. Overall, the LC ordering involved in the synthesis of materials provides unique optical properties (anisotropy) , responsive properties (upon heating, exposure to light or 8,24

chemicals) stimuli)

3,29–36

, and shapes (both as in the final shape and the change in response to an external

25–28

that would be possible with the presence of LC media.

There have been successful applications demonstrated for polymeric materials synthesized from the LC media. Examples of such polymers include self-cleaning surfaces with oscillating surface topology upon an external electric field , liquid-release media that responds upon exposure to 37

light , light-driven actuators 38

, molecular motors

3,29,39–41

30,40

and artificial cilia . These examples clearly 42

demonstrated that the bulk response of the polymers could be obtained which originated from externally-triggered molecular-scale events occurring within the anisotropic media. Beyond their anisotropic optical and responsive properties, studies have also revealed that porosity can also be incorporated into the polymers during the synthesis. Nanoporous polymeric materials synthesized from LCs, in which usually lyotropic LCs (hexagonal, lamellar, columnar or cubic phase) or thermotropic smectic phases (due their layered structure) was employed as templates during the synthesis.

43–49

Since these LC phases require certain conditions and chemicals

to form, the synthesis methods were limited to those conditions. For PDLCs, LC-rich and polymerrich phases have been formed within the polymeric network through demixing during polymerization. The morphology of the LC-rich domains is observed to be of usually disconnected,


3


spherical domains within polymer-rich network.

13,50–52

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Polymerization of the aqueous emulsions of

reactive and non-reactive mesogen mixtures resulted in microparticles with mesoscale porosity.

7,19

Interestingly, the porosity was evident in the microparticles that were synthesized from LC droplets with radial configuration that did not exhibit substantial shrinkage upon extraction of the non-reactive mesogens. The reason of this non-shrinking behavior was considered to be due to the symmetry of the radial configuration that did not allow a major shrinkage of the microparticles upon extraction. Besides these, studies involving polymerization of LC shells demonstrated similar morphologies . Overall, synthesis methods that were described to date were usually based on 53

certain geometries or certain types of LCs. Thus, there is a need for a more generalizable and scalable synthesis methods that allow control over the ordering and porosity at the nano and meso scale. The motivation of this study was designing a method for the synthesis of mesoporous membranes that go beyond the currently available examples by providing a facile, scalable and a universal tool that can be employed to a wide range of LC phases. We report characterization of the polymeric films (of areas in the order of 1-10 cm ) synthesized from mixtures of reactive and 2

non-reactive mesogens. We found that porosity can be incorporated into the polymers if the area of the polymers were restricted during the extraction of the non-reactive mesogens from the polymerized mixtures. We showed that this area restriction can be maintained in two different ways; (i) through a physical constraint (such as covalently bonding the polymer to the underlying substrate or mechanically preventing the films from shrinkage), or (ii) a symmetrical constraint (such as using LCs with chiral symmetry during the synthesis). Upon application of such constraints, we observed that open pores of average diameters in the range 10-40 nm could be formed (evidenced both by electron microscopy and nitrogen adsorption isotherms), which can be


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tuned by varying the concentration of the reactive mesogens. In addition, the method that we develop here also provides control over the direction of the pores through ordering of the LCs. We hypothesized that this directionality can provide advantages of the mass transfer during the applications, which we successfully demonstrated in ultrafiltration of aqueous solutions for proteins and nanoparticles.

2. Experimental Section Materials: 4-cyano-4!-pentylbiphenyl (5CB), E7 and 4-(3-acryloyoxy-propyloxy) benzoic acid 2-methyl-1,4-phenylene ester (RM257) were purchased from HCCH Jiangsu Hecheng Chemical Materials

Co.,

Ltd.

Polyvinyl

alcohol

(PVA),

3-(trichlorosilyl)propyl

methacrylate,

dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP), photoinitiator (1hydroxycyclohexyl phenyl ketone), anhydrous methanol, anhydrous acetone and toluene were obtained from Sigma-Aldrich. Anhydrous ethanol was purchased from J.T. Baker. Perfluorodecyltrichlorosilane (PFDTS) was purchased from Alfa Aesar. 3-(trichlorosilyl)propyl methacrylate (TCSPM) was purchased from TCI Chemicals (Japan). Sulfate-coated 200 nm-indiameter

polystyrene

particles

were

obtained

from

Life

Technologies.

4-(1-

methylheptyloxycarbonyl)phenyl-4-hexyloxybenzoate) (S-811) was purchased from Merck. Glass slides were obtained from Marienfeld (Germany). Preparation of Liquid Crystal-Monomer Mixtures: For the mixtures of liquid crystal and monomer, non-reactive mesogen 5CB and the reactive LC monomer RM257 was added at 10%40% wt/wt. Also, the photoinitiator 1-hydroxylcyclohexyl phenyl ketone was added at 5% wt/wt based on the mass of RM257 to the mixture. For chiral LC mixtures, S-811 was added as chiral dopant and a liquid crystal mixture, E7, was used due to its higher nematic-isotropic phase


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transition. In order to solve the liquid crystal and prepare a homogenous solution, toluene was added as a solvent and mixed using vortex mixer until a clear solution was obtained. Then, toluene was evaporated from the solution under vacuum. Cleaning of the glass surfaces: Before functionalization of the glass surfaces by DMOAP, PFDTS, or TCSPM, O plasma was applied to the glass slides using a Diener Electronics, Zepto 2

Plasma Unit. O was fed to the system with a flow rate of 100 cm /min for 15 minutes for the 3

2

plasma cleaning. Functionalization of the glass surfaces: The surfaces of the glass slides were functionalized with different chemicals. Polyvinyl alcohol (PVA) was used for the planar anchoring. The glass slides were coated by the spin coater (5000 rpm for 2 minutes) with 5% wt PVA in water solution using a Polos Spin Coater System. Perfluorodecyltrichlorosilane (PFDTS), Dimethyloctadecyl[3(trimethoxysilyl)propyl]ammonium chloride (DMOAP), or 3-(trichlorosilyl)propyl methacrylate (TCSPM) were used for the homeotropic anchoring. PFDTS and TCSPM were deposited on the surface with overnight incubation in a vacuum chamber. DMOAP was deposited on the glass surfaces from 10 minutes incubation in its 1% wt aqueous solution. Preparation of polymeric films: Sandwich cells were prepared with two pieces of functionalized glass slides. The spacing was set using a polyethylene film as a spacer (Saran wrap). Prepared glass slides were filled with LC-monomer (and chiral dopant for cholesteric LCs) mixture with the help of capillary action. Then, the photopolymerization of the film was done by using Spectroline ENF-280C/FE UV lamp (365 nm; Westbury, NY) that delivered 2.5 mW.cm at a distance of 5 -2

cm from the light source. Sandwich cells were exposed to UV light for 30 minutes. After the polymerization, polymer films were taken from the surface of the PFDTS coated glass slides by using razor blade. For the PVA coated glass slides, sandwich cells were placed into petri dishes


6

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which were filled with distilled water and incubated overnight to dissolve PVA. When PVA that coated the surface of the glass slides were dissolved, polymer films were spontaneously detached from the surfaces. Analysis of the shrinkage: Extraction of 5CB from the synthesized polymeric films was done using excess amount ethanol, acetone or toluene. Change in the size of the polymer films after the extraction was observed under the optical microscope and percent shrinkage was calculated based on the size of the polymer films before and after the extraction. Optical characterization: Optical characterizations of the films were performed using an Olympus BX50 microscope (Olympus Inc., Japan) equipped with a polarizer and an analyzer filters. Alignment of the polymers were determined from polarized micrographs in transmission mode. Characterization of morphology of the polymeric film: The morphology of the polymeric films was characterized by using QUANTA 400F Field Emission series scanning electron microscope (SEM). For the preparation of the samples, polymer films or polymers covalently bonded on glass slides were cut into a sufficient size. For the characterization of the average pore size and accessibility of the pores, nitrogen adsorption porosimetry was used. The N adsorption-desorption 2

isotherms were obtained by using Micromeritics Tristar II instruments at 77 K. The samples were prepared as covalently bonded to the substrates functionalized with TCSPM. Their alignments were checked by optical microscopy and the pore structures were checked by SEM prior to adsorption studies. The samples were degassed under vacuum at 333 K overnight. Barrett–Joyner– Halenda (BJH) model was used to estimate the average pore size of polymeric films based on the desorption data.


7


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Thermal analysis: Differential Scanning Calorimetry (DSC) (Shimadzu DSC-60) was used to characterize the thermal behavior of the polymer, heating and cooling was applied to the polymer between 293 K and 473 K at a rate 5 K/min. In addition, Thermal Gravimetric Analysis (TGA) (Shimadzu DTG-60H) was applied to the polymer by heating the sample from room temperature to 873 K with a heating rate of 10 K/min and measuring the change in the weight of the sample. Tensile Tests: Tensile tests were performed using a Shimadzu Autograph AG-100 KNIS MS universal tensile testing instrument. The samples were prepared in the form of films in dimensions of 30 mm gauge length, 10 mm in width and 120 µm in thickness following ASTM standards ASTM-D882. The tests were performed in a strain rate of 3 mm/min at room temperature. Filtration: Filtration tests were performed in filter modules connected to the syringe. A transmembrane pressure difference of 1 bar was applied during filtrations. Ethanol, water, solution of bovine serum albumin (BSA) in phosphate-buffered saline (1g/L) or solution of 200 nm- in diameter sulfate PS in water were used as feed and permeates were collected with respect to time to calculate permeances (L/m h.bar) and rejection percentage. In order to obtain the concentration 2

of BSA in the permeate, UV-visible spectrophotometer (Shimadzu UV-1601) was used.


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3. Results and Discussion We used mixtures of a reactive mesogen, 4-(3-acryloyloxypropyloxy) benzoic acid 2-methyl-1, 4-phenylene ester (RM257) and a non-reactive mesogen, 4-cyano-4!-pentylbiphenyl (5CB) (Figure 1a). The mixtures were sandwiched between two functionalized glass surfaces that mediate a determined alignment of the mesogens. Then, the mixtures were photopolymerized using a UV-light and the sandwich cells were detached to obtain free standing polymeric films (Figure 1b). Following this procedure, we were able to synthesize polymeric films of size in the order of 1-10 cm (Figure 1c). We used rubbed-polyvinylalcohol (PVA) surfaces for uniform 2

planar alignment and perfluorodecyltriethoxysilane (PFDTS)-deposited glass surfaces for the homeotropic alignment of the mesogens. Using surfaces of different chemical functionality during the synthesis, we were able to provide control over the alignment of the resulting polymeric network as determined by optical microscopy. Figure 1d shows the polarized light optical micrographs of two separate 20 µm-thick films synthesized from!20% wt RM257 between two surfaces of planar alignment (top row) and two surfaces of homeotropic alignment (bottom row). The films synthesized between rubbed-PVA surfaces exhibitted dark appearance under polarized light when the average alignment of the LCs in the direction of one of the polarizers. When the film was rotated 45 degrees, the appearance of a bright transmitted light was consistent with the uniform planar alignment. The films synthesized between PFDTS-coated surfaces did not exhibit such an angle-dependent appearance under polarized light and the cross pattern was visible in the conoscopic images, which are two evidences consistent with a uniform homeotropic alignment of the LCs. In addition, we observed that such an alignment is preserved within the films up to a thickness of 160 µm as evidenced in Figure S1.


9



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(mainly 5CB) was removed completely from the polymer, and the mass of the polymer left was consistent with the amount of the reactive monomer initially mixed with 5CB (Figure S2 – S4). We also note that the FTIR data of the reactive and non-reactive mesogen, their mixtures, and the polymer before and after extraction were collected and their signatures were consistent with similar characterizations reported in the literature (detailed in Figure S2).

54

After the extraction of the unreacted mesogens and drying of the solvent, we observed an anisotropic shrinkage of the films. Figure 2 shows the micrographs of the 40 µm-thick films synthesized from 20% wt RM257 in 5CB before and after the extraction of the unreacted mesogens. Figure 2a shows the micrographs of a piece of a film synthesized between surfaces of planar anchoring, where the nematic director was towards the short axis (shown as n in the figure). After the extraction, a substantial shrinkage of the film in a direction perpendicular to the director is evident in the micrographs. In addition, the alignment within the film was preserved after the extraction step as evidenced by the polarized light micrographs shown in the right panel of Figure 2a and b. To provide more evidence on the director-dependent shrinkage of the films, we performed the same optical characterization to two more films synthesized with different alignment conditions at the two surfaces (Figures 2b and c). Consistent with the observation of a nematic director-dependent shrinkage, we observed isotropic shrinkage of the films that were synthesized between surfaces of homeotropic anchoring and a curling of the film into a cylindrical shape when the films were synthesized between surfaces mediating homeotropic-planar hybrid anchoring at the two sides. Cylindrical shape was formed due to a shrinking imbalance between the two surfaces. These observations led us to perform characterizations on the structure of the films synthesized from liquid crystalline molecular templates since the shrinking behavior of the


11



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Page 13 of 37

shrinkage of the polymers with respect to their initial size as a function of the monomer concentration. A linearly decreasing percent shrinkage of the films from 45% to 25% was observed perpendicular to the director as the RM257 content of the films were increased from 10% to 30% wt, whereas nearly 7% shrinkage was observed in the direction perpendicular to the director, independent of the RM257 content of the films. This shrinking behavior is consistent with the previous observations in polymerized LC droplets by Wang et al. Along with these observations, 19

when the changes in the volumes of the films were calculated from the measured dimensions, we observed a linear dependency of the final to initial volume ratio (V /V ) as a function of the RM257 f

i

content. a)

50 45

perpendicular

40

parallel

% Shrinkage

35 30 25 20 15 10 5 0 0

5

10

15

20

25

30

35

10 15 20 25 RM257 content (% wt)

30

35

RM257 content (% wt)

b) 0,7 0.7

0,6 0.6 0,5 0.5 Vf/V i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,4 0.4 0,3 0.3 0,2 0.2 0,1 0.1 0 0

5

Figure 3. (a) Percent shrinkage of the polymeric films along (open symbols) and perpendicular (filled symbols) to the director as a function of the concentration of RM257 after extraction of


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5CB. (b) Ratio of volumes of films before (V ) and after (V ) extraction of 5CB as a function of the i

f

concentration of RM257. Analysis were done on films with thickness of 160 "m.

When the films were placed in the solvent for extraction, we did not observe shrinkage of the films even after overnight incubation of the films in the solvent (although the solvent dissolved the entrapped unreacted mesogens). Interestingly however, a substantial shrinkage occurred immediately after the films were dried out from the solvent used for extraction. Therefore, the two observations, (i) a shrinkage of the films that correlate with the reactive mesogen content (Figure 3b) and (ii) the occurrence of the shrinkage immediately and only after the films were dried from the extraction solvent, evidences an incorporation of porosity within the films, which close after drying following the extraction. We note that, supporting this hypothesis, we observed swelling of the dried films back to their initial dimensions when exposed to toluene and acetone, the solvents that we found to plasticize the polymerized RM257 (Figure S5). However, no substantial effects of water or ethanol exposure on the size or shape of the films were observed after exposure. We relate this behavior to the reversible closure of the pores incorporated to the polymer structure after extraction. When we examine the scanning electron microscopy (SEM) images of the films after extraction, we observed rough surfaces both for the films synthesized between PVA-coated and PFDTScoated glass surfaces (Figure 4a and b). Since the PVA or PFDTS-coated glass surfaces would not potentially incorporate such roughness to the surfaces of the polymers, we hypothesize the rough surface features evident in the SEM micrographs to result from the closure of the pores within the films.


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reactive underlying substrate, we functionalized the glass surfaces with acrylate-terminated silane molecules, 3-(trichlorosilyl)propyl methacrylate (TCSPM), and synthesized the polymer via polymerization of the RM257-5CB mixture on the surfaces (Figure 5a). When this procedure was followed, we did not observe polymer to detach from the TCSPM-functionalized substrate and the polymer did not shrink after extraction of the unreacted mesogens or drying of the films from the solvent used for extraction. We found that the prevention from shrinkage was independent of the top substrates contacting the polymer during synthesis (either PFDTS-functionalized or PVAcoated glass), which were removed after polymerization. These observations suggested the pores to maintain their openings after the extraction step. When the SEM micrographs of the extracted films covalently attached to the TCSPM substrates were compared to the micrographs of the shrunk films, the pores were evident in the covalently attached films (Figure 4c and d). This observation is consistent with our above hypothesis that the shrinkage of the films was due to the destruction of the pores. Here we also note that we did not observe the alignment of the polymer to change after stripping the polymer off from the TCSPM surface after polymerization (Figure S6).


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Next, we characterized the microstructure of the pores incorporated into the films as a function of RM257 loading. We used SEM and nitrogen adsorption isotherms to reveal details about the pore structure of the film. We prepared polymeric films of varying RM257 content between TCSPM-PFDTS substrates and collected SEM micrographs shown in Figure 6a. The films exhibited different morphologies and pore sizes depending on their RM257 content. Films of 10% wt RM257 exhibited a fibrous structure with large interconnected pore structure, whereas films prepared from RM257 concentration in the range 20-40% wt, exhibited more disconnected, smaller size pores. We characterized the pore size distributions based on the collected SEM micrographs as shown in Figure 6b. As expected, the width of the pore size distribution was substantially wider for the films of 10% wt RM257 (where we averaged the sizes of the inner pores that were away from the surfaces) compared to the other films. The average sizes of the pores calculated from these histograms were found to be linearly dependent on the RM257 content of the films. The average pore diameter decreased from 40 nm to 10 nm as the RM257 content was increased from 10% to 40% by weight. We also note that the average pore diameters in the films were similar to the average pore diameters incorporated into the polymeric particles synthesized from aqueous emulsions of LC-monomer mixtures. Consistent with these SEM characterizations, 19

the average pore sizes calculated from nitrogen sorption isotherms were found to be 29.6 ± 2.8 nm and 24.4 ± 0.7 nm for the films synthesized from 20% wt RM257 and 40% wt RM257, respectively. Nitrogen sorption isotherms also showed that the mesoscale pores evident in the micrographs were accessible to the external species. Formation of mesoscale pores were consistent with the polymerization-induced phase separation generally observed for polymer dispersed liquid crystals reported in previous studies.

56,57

Consistent with the formation of the mesoscale pores, we

observed the turbidity of our films to increase after photopolymerization (Figure S8). We also note


18

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Figure 6. Characterization of the pore sizes of the polymeric films synthesized from mixtures of RM257 and 5CB. (a) Scanning Electron Micrographs of the polymeric films synthesized form different concentrations of RM257. Scale bars: 500 nm. (b) Pore size distribution of the films measured using SEM micrographs, (c) Nitrogen sorption isotherm (left) and the calculated pore size distribution using BJH theory (right) of the film synthesized from 20% RM257 and 80% 5CB by weight.

In addition to the covalent bonding to the surfaces, prevention from shrinkage could also be maintained by placing a restriction through ordering symmetry of chiral nematics. Since the shrinking behavior of the films were director-dependent, and more preferential in a direction perpendicular to the local nematic director, the ordering symmetry of chiral nematics, when chemically crosslinked, would prevent the shrinkage in the nematic director. Figure 7 shows an SEM of a film synthesized from an LC mixture with 20% wt chiral dopant, S-811, between rubbedPVA surfaces causing planar anchoring. The image shows the surface of a free-standing film (not bonded on the underlying substrate) after extraction of the unreacted mesogens. The pores are evident in the images, which are in the range 151 ± 34 nm. We also observed an insignificant shrinkage of 5% through the thickness of the films. This result and generalization approach also explained the incorporation of porosity to the polymeric films synthesized from hydrogen-bonded cholesteric networks after evaporation of the templating material.

58,59

Here we also note that the

collapse of the pores can also be prevented by placing the films between two surfaces that provide a physical constraint (such as between two o-rings). An example of this is shown below along with a functional property of the films.


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Figure 7. Scanning electron micrograph of the surface of a free-standing, 20 "m-thick polymeric film synthesized from 20% RM257, 20% chiral dopant and 60% 5CB by weight (after the extraction of E7 with the excess amount of ethanol). Scale bar: 2 "m.

Here, we must highlight a more interesting property of the pores. When the pore structures of the 20% wt RM257 films synthesized between TCSPM-PFDTS or TCSPM-PVA sandwich cells were compared, a substantial difference in the morphology of the pores is evident in the SEM micrographs (Figure 4c, d and S4). The films synthesized between TCSPM-PFDTS surfaces exhibit pores which had no substantial directionality along the surface plane, whereas the films synthesized between TCSPM-PVA surfaces exhibit an overall in-plane orientation in the rubbing direction of the PVA substrate (larger scale images are shown in Figure S9, and films with higher monomer concentration is shown in Figure S10.). These observations were further supported by the Fourier Transform of the images included as insets to the corresponding SEM micrographs. If


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the orientations of the pores were determined by the nematic director, we expected the films synthesized between TCSPM-PFDTS to exhibit an overall orientation of the pores in a direction orthogonal to the film surfaces. Supporting this expectation, the SEM micrographs of the crosssection of the homeotropic films synthesized between TCSPM-PFDTS substrates indeed exhibited a pore directionality orthogonal to the film surface planes as shown in Figure 8a. Moreover, such a directionality was absent in the cross-section images of the films synthesized from chiral LC mixture (Figure 8b). FFT of the images shown in insets are also supporting this anisotropy. a)

n

b)

Figure 8. Scanning electron micrographs of the cross-section of 20 "m-thick polymeric films synthesized from (a) 20% RM257 and 80% 5CB by weight between TCSPM-PFDTS substrates, and (b) 20% S-811, 20% RM257 and 60% E7 by weight between PVA substrates. n showing


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nematic director. The film shown in (a) is covalently bonded to the underlying substrate, whereas the film in (b) is a free-standing film. The PFDTS and PVA substrates were removed for extraction of the films. Scale bars: 10 "m. Insets shows the FFT the of the corresponding images.

We have evidenced that the films synthesized from LC templates exhibit mesoscale pores in a direction determined by the nematic director during the synthesis. We hypothesize that we can employ this control over the microstructure of the films to significantly improve the performance of the films that are to be used in chemical processes. We end this paper by showing a proof of concept demonstration of a functional property of the films synthesized from liquid crystals that provide advantageous mass transfer properties. The accessibility of the pores and an average pore size in the order of 10 nm suggests the use of the films as ultrafiltration membranes. However, a film is needed to be defect (hole)-free in order to be used as a membrane. As a conceptual illustration, we prepared films from 20% wt RM257 in 5CB with homeotropic and planar ordering. We prevented the films from shrinkage and measured the permeances of ethanol, water, and aqueous solutions of bovine serum albumin (BSA) and 200 nm-in-diameter polystyrene (PS) particles. In addition, we calculated the rejection of BSA and PS particles for the quantification of the separation performance. The motivation behind this set of experiment was three folds; (i) to examine whether there are defects (holes) within the polymeric films that were not quantified in N adsorption, optical or electron micrographs, (ii) to demonstrate that the directions of the pores 2

has an influence on the permeances through the films, and (iii) to examine whether the pores can serve as a separation medium for molecular-level species (1-10 nm). As shown in Table 1, during the filtration of PS particles with 200 nm in diameter from water, we did not observe any permeated PS particles through the films, which suggests that there were


23


Page 24 of 37

no holes within the films that would allow particles in the 100 nm range to permeate. Second, when the permeances of water and ethanol were compared, we observed up to two orders of magnitude higher permeances through films with pores orthogonal to the surfaces compared to their horizontal counterparts. Ethanol permeance were observed to be 24.7 #!4.0 L/m .h.bar for 2

planar films, whereas it was observed to be 602.8 #! 178.1 L/m .h.bar for homeotropic films. 2

Similarly, water permeance were observed to be 1.6 #!0.1 L/m .h.bar for planar films, whereas it 2

was observed to be 658.5 #!211.1 L/m .h.bar for homeotropic films. This supports our hypothesis 2

about the mass transfer properties of the films were significantly improved when the pores were along to permeation direction, and the permeance values observed became comparable or superior over the water permeances observed for ultrafiltration membranes reported in the literature.

60–63

Lastly, films exhibited 78.4% #!3.4% rejection of BSA for planar films whereas 37.5% #!2.6% rejection was observed in homeotropic films. Observation of these rejections during the filtration of the aqueous solutions of BSA, which clearly suggests that these films can potentially be developed as membranes for ultrafiltration purposes.


24

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Table 1. Results of the filtrations tests performed using 80 "m-thick films synthesized from 20% RM257 and 80% 5CB, by weight.

Horizontal Pores

Orthogonal Pores

Permeance (L/m .h.bar)

Rejection (%)

Permeance (L/m .h.bar)

Rejection (%)

Pure Ethanol

24.7 #!4.0

-

602.8 #!178.1

-

Pure Water

1.6 #!0.1

-

658.5 #!211.1

-

1 g/L BSA in PBS

0.6 #!0.1

78.4 #!3.4

5.5 #!2.0

37.5 #!2.6

200 nm-in-diameter PS in water

2.4

ND (~100%)

4.5 #!1.8

ND (~100%)

Feed

2

2

Overall, we characterized the microstructures of the films templated from liquid crystalline media. We found that pores of an average diameter of 10-40 nm with controlled directionality via LC director can be incorporated into the films, when constrained in a determined area. We showed that this area constraint can be succeeded either by covalently bonding the film to the supporting surfaces, determining the area by external surfaces (o-rings), or due to the inherent symmetry of the liquid crystalline phases (such as chiral liquid crystal phases). As a proof-of-concept study, we demonstrated that the films synthesized from the polymerization of the mixtures of reactive and non-reactive mesogens can potentially be used as ultrafiltration membranes. We showed that these films exhibit a consistent separation of the feed solutions and the permeances that depend on the direction of the pores. Here we note that we left the more detailed characterization of the ultrafiltration performance of the films to our future studies.


25


Page 26 of 37

4. Conclusion The results reported in this study provide methods for the synthesis of porous polymeric films of area in the order of 1-10 cm using liquid crystals as templates. Specifically, we showed when 2

photopolymerized films of reactive and non-reactive mesogen mixtures were constrained in an area during the extraction of the unreacted mesogens, open pores were incorporated into the polymers. We characterized the morphologies of such films and found that the average diameter of the pores was in the range 10-40 nm, and can be tuned by varying the reactive monomer concentration. In addition, the method that we developed herein, provides control over the average direction of the pores through varying the nematic director, which suggest potential mass transfer advantages. Overall, the method presented in this study provides control over the pore size and directionality that would be difficult to achieve through the use of isotropic fluids. The methods that were developed in this study are generalizable. That is, the methods can also be applicable to other liquid crystalline phases such as cholesterics, blue phases, biaxial nematics etc., which would provide a rich palette of three-dimensional pore structures that would be advantageous in a scientific and technological perspective. The morphology of the films synthesized from liquidcrystal templates were also successfully demonstrated to be useful as an ultrafiltration membrane. A number of future directions could be suggested based on the findings in this study, some of which include characterization of the ultrafiltration performance of the membranes, functionalization of the pores (such as acidic or basic groups) and tuning the structure and geometry of the pores through the ordering symmetry of the liquid crystals.


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ASSOCIATED CONTENT Supporting Information. Thermal analysis (DSC and TGA), tensile test results and additional SEM and brightfiled optical micrographs of the polymeric films are provided as supporting information. AUTHOR INFORMATION Corresponding Author *to whom the correspondence should be addressed. [email protected] FUNDING SOURCES This work was partially supported by TÜB"TAK-B"DEB 2232 under award number 116C093. ACKNOWLEDGMENT The authors thank Dr. Erhan Bat, Dr. Pınar Zeynep Çulfaz-Emecen and Dr. Önder Özbelge for their helpful discussions. REFERENCES (1)

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